Calculating Magic Hit Chance

Can someone explain where I'm going wrong with my spell hit chance calculation?

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I use the following formulas:

HC = Hit Chance

BHC = Base Hit Chance

SA = Spell Accuracy

MD = Monster Magic Defense

Hit% = HC/200

HC = BHC + SA - MD

If an enemy is weak to an element, 40 is added to HC. If an enemy is strong to an element, BHC = 0.

In my example, I'm using Kraken1 as my opponent and the Brak spell with a SA of 64. Kraken1 has no weakness or strength to poison. Kraken1 MD = 80

HC = 148 + 64 - 80 = 132

Hit% = 132/200 = 66%

Now, for Kraken2 with an MD of 100

HC = 148 +64 - 100 = 112

Hit% = 112/200 = 56%

So my calculations say for K1 and K2, Brak has a 66% and 56% chance of hitting, respectively. When I look it up, it says that for K1 and K2, you have a 26% and 6% respectively, which seems much more reasonable.

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Also, I've heard several times that Brak has a 3/256 chance to hit Chaos. How does this work? My calculation would be:

HC = 0 (strong against everything) + 64 - 100 = -36.

So if it's a negative number, it should be a zero chance of this hitting?